Effects of macropore size in carbonate apatite honeycomb scaffolds on bone regeneration.

The pore architecture of scaffolds is a critical factor for angiogenesis and bone regeneration. Although the effects of scaffold macropore size have been investigated, most scaffolds feature macropores with poor uniformity and interconnectivity, and other parameters (e.g., microporosity, chemical composition, and strut thickness) differ among scaffolds. To clarify the threshold of effective macropore size, we fabricated honeycomb scaffolds (HCSs) with distinct macropore (i.e., channel) sizes (~100, ~200, and ~300 µm). The HCSs were composed of AB-type carbonate apatite with ~8.5% carbonate ions, i.e., the same composition as human bone mineral. Their honeycomb architecture displayed uniformly sized and orderly arranged channels with extremely high interconnectivity, and all the HCSs displayed ~100-µm-thick struts and 0.06 cm3 g-1 of micropore volume. The compressive strengths of HCSs with ~100-, ~200-, and ~300-µm channels were higher than those of reported scaffolds, and decreased with increasing channel size: 62 ± 6, 55 ± 9, and 43 ± 8 MPa, respectively. At four weeks after implantation in rabbit femur bone defects, new bone and blood vessels were formed in all the channels of these HCSs. Notably, the ~300-µm channels were extensively occupied by new bone. We demonstrated that high interconnectivity and uniformity of channels can decrease the threshold of effective macropore size, enabling the scaffolds to maintain high mechanical properties and osteogenic ability and serve as implants for weight-bearing areas.

Fabrication of carbonate apatite honeycomb and its tissue response.

Carbonate apatite (CO3 Ap) block can be used as a bone substitute because it can be remodeled to new natural bone in a manner conforming with the bone remodeling process. Among the many porous structures available, honeycomb (HC) structure is advantageous for rapid replacement of CO3 Ap to bone. In this study, the feasibility to fabricate a CO3 Ap HC was studied, along with its initial tissue response in rabbit femur bone defect. First, a mixture of Ca(OH)2 and a wax-based binder was extruded from a HC mold. Then the fabricated HC was heated for binder removal and carbonation at 450°C in a mixed O2 -CO2 atmosphere, forming a CaCO3 HC. When the CaCO3 HC was immersed in 1 mol/L Na3 PO4 solution at 80°C for 7 days, its composition changed from CaCO3 to CO3 Ap, maintaining the structure of the original CaCO3 HC. Compressive strengths of the CaCO3 and CO3 Ap HCs were 65.2 ± 7.4 MPa and 88.7 ± 4.7 MPa, respectively. When the rabbit femur bone defect was reconstructed with the CO3 Ap HC, new bone penetrated the CO3 Ap HC completely. Osteoclasts and osteoblasts were found on the surface of the newly formed bone and osteocytes were also found in the newly formed bone, indicating ongoing bone remodeling. Furthermore, blood vessels were formed inside the pores of CO3 Ap HC. Therefore, CO3 Ap HC has good potential as an ideal bone substitute. © 2019 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 1014-1020, 2019.

Fabrication of octacalcium phosphate block through a dissolution-precipitation reaction using a calcium sulphate hemihydrate block as a precursor.

Although octacalcium phosphate (OCP) powder and a collagen/gelatin composite demonstrate good potential as bone substitutes, an OCP block has not been fabricated to date. In this study, the feasibility of fabricating an OCP block was evaluated through a dissolution-precipitation reaction using a calcium sulfate hemihydrate (CSH) block as a precursor. When the block was immersed in a phosphate salt solution, its composition changed to that of OCP, while its structure was maintained. The diametral tensile strength (DTS) of the OCP block was 1.0 ± 0.2 MPa. The macroporosity and microporosity of the OCP block were 33.4 ± 4.5% and, 69.0 ± 1.6%, respectively. New bone attached well to the OCP block, and this block was partially replaced by bone 2 weeks after implantation. Four weeks after implantation, the surface of the OCP block was nearly covered with new bone and ~30% of the block was replaced by new bone, while no replacement by bone was observed in the case of a hydroxyapatite (HAp) block used as a control. It is concluded that OCP blocks are potentially suitable for their use as artificial bone substitutes.

Hydrothermal treatment for TiN as abrasion resistant dental implant coating and its fibroblast response.

Dental implant made of pure titanium (Ti) is prone to scratch and abrasion during routine oral hygiene procedures. This results an increase in surface roughness and therefore, facilitates the adhesion of bacteria. In severe cases, this could lead to peri-implantitis. To overcome this problem, surface modification of Ti is necessary to improve its abrasion resistance. Besides, a strong implant-gingiva interface should also be guaranteed to prevent the adhesion of bacteria. In this study, titanium nitride (TiN) coating was first prepared with gas nitriding to increase surface hardness of pure the substrate. Then, the TiN was hydrothermally treated in CaCl2 solution in order to improve its soft tissue biocompatibility. The effect of hydrothermal treatment temperature on surface properties of TiN was investigated and its biocompatibility was assessed in vitro using NIH3T3 fibroblast cell. It was determined that 120°C was the critical temperature for the hydrothermal treatment condition. Treatment below 120°C could incorporate Ca into TiN surface, oxidize TiN surface partially and then improve the wettability while preserving its morphology and hardness. Fibroblast cell attachment and proliferation were improved and cell spreading was enhanced on hydrothermally treated specimens compared with untreated ones. Improved wettability, Ca incorporation and negative surface due to interstitial N were believed to be the main reasons. Hydrothermal treatment is expected to make TiN a promising dental implant coating with excellent abrasion resistance and good soft tissue affinity.

Effects of PLGA reinforcement methods on the mechanical property of carbonate apatite foam.

The purpose of this study was to improve the mechanical property of brittle carbonate apatite (CO3Ap) foam aimed as bone substitute material by reinforcement with poly(DL-lactide-co-glycolide) (PLGA). The CO3Ap foam was reinforced with PLGA by immersion and vacuum infiltration methods. Compressive strength of CO3Ap foam (12.0±4.9 kPa) increased after PLGA reinforcement by immersion (187.6±57.6 kPa) or vacuum infiltration (407.0±111.4 kPa). Scanning electron microscopic (SEM) observation showed a gapless PLGA and CO3Ap foam interface and larger amount of PLGA inside the hollow space of the strut when vacuum infiltration method was employed. In contrast a gap was observed at the PLGA and CO3Ap foam interface and less amount of PLGA inside the hollow space of the strut when immersion method was employed. Strong PLGA-CO3Ap foam interface and larger amount of PLGA inside the hollow space of the strut is therefore the key to higher mechanical property obtained for CO3Ap foam when vacuum infiltration was employed for PLGA reinforcement.

Influence of PLGA concentrations on structural and mechanical properties of carbonate apatite foam.

Reinforcement with bioresorbable polymer such as PLGA is one of the useful ways to improve the mechanical property of brittle carbonate apatite (CO3Ap) foam. In the present study, CO3Ap foam was reinforced with various concentrations of PLGA solution (5, 10, 15 and 20 wt%) using vacuum infiltration method and its influence on structure, porosity and mechanical property was investigated. It was found that the amount of PLGA inside the hollow space of the CO3Ap foam strut increased with the concentration of PLGA. Porosity likewise was significantly (p<0.05) reduced from 94% (CO3Ap foam without PLGA) down to 82% (CO3Ap foam reinforced with 20 wt% PLGA). Compressive strength of CO3Ap foam significantly (p<0.05) increased from 0.02 MPa (CO3Ap foam without PLGA) up to 1.5 MPa (CO3Ap foam reinforced with 20 wt% PLGA).

Effect of precursor's solubility on the mechanical property of hydroxyapatite formed by dissolution-precipitation reaction of tricalcium phosphate.

The effect of the solubility of the precursors, alpha tricalcium phosphate (α-TCP) and beta tricalcium phosphate (ß-TCP) on the mechanical strength of hydroxyapatite (HAp) bone substitute was investigated. Uniaxially pressed block starting from these precursors were treated hydrothermally with 1 mol/L of ammonia solution at 200°C for various durations. XRD analysis revealed that α-TCP block took 3 h whereas ß-TCP block took 240 h for complete transformation to HAp. The porosity of HAp obtained from ß-TCP block was found to be lower than that of HAp from α-TCP block. Diametral tensile strength of HAp from ß-TCP block showed a significantly higher value than that of HAp from α-TCP block. It is therefore concluded that solubility of precursor affects the mechanical strength of the HAp block.

Fabrication of microporous calcite block from calcium hydroxide compact under carbon dioxide atmosphere at high temperature.

Effects of carbonation temperature and compacting pressure on basic properties of calcite block were studied using Ca(OH)2 compact made with 0.2-2.0 MPa and their carbonation at 200-800ºC for 1 h. Microporous calcite was obtained only when carbonated at 600ºC using Ca(OH)2 compact made with 0.2 MPa even though thermogravimetry analysis showed that calcite powder was stable up to 920ºC under CO2 atmosphere. CaO formed by carbonation at 700ºC and 800ºC is thought to be caused by the limited CO2 diffusion interior to the Ca(OH)2 compact. Also, unreacted Ca(OH)2 was found for Ca(OH)2 compact prepared with 0.5 MPa or higher pressure even when carbonated at 600ºC. As a result of high temperature carbonation, crystallite size of the calcite, 58.0 nm, was significantly larger when compared to that of calcite prepared at room temperature, 35.5 nm. Porosity and diametral tensile strength of the microporous calcite were 39.5% and 6.4 MPa.

Effects of sintering temperature on physical and compositional properties of alpha-tricalcium phosphate foam.

Effects of sintering temperature on the physical and compositional properties of alpha-TCP foam fabricated using the polyurethane foam method were examined. When a polyurethane foam coated with alpha-TCP slurry was sintered at 1,400-1,550 degrees C, alpha-TCP foam having basically the same fully interconnected porous structure was produced although shrinkage occurred with increasing sintering temperature. On porosity of the alpha-TCP foam, a higher foam porosity of 95% was obtained when sintered at 1,400 degrees C as compared to the 90% porosity obtained at a higher sintering temperature of 1,550 degrees C. Further, at 1,500 degrees C or higher temperature, frame became dense with disappearance of micropores. On compressive strength, it increased from approximately 50 to 250 kPa when sintering temperature was increased from 1,400 to 1,550 degrees C. Nonetheless, no compositional changes were observed even when the alpha-TCP foam was cooled in the furnace without quenching process. In light of the results obtained, it was concluded that alpha-TCP foam fabricated using the polyurethane method was useful as a bone substitute and/or scaffolding material for tissue engineering. Besides, alpha-TCP foam could be useful as a precursor for the fabrication of other calcium phosphate foams.

Effect of temperature on crystallinity of carbonate apatite foam prepared from alpha-tricalcium phosphate by hydrothermal treatment.

The effect of temperature on crystallinity of carbonate apatite (CAp) foam prepared from alpha-tricalcium phosphate (alpha-TCP) foam by hydrothermal treatment was investigated in the present study. The alpha-TCP foams were prepared through a conventional sintering method using polyurethane foam as template. Then, the resultant alpha-TCP foams were hydrothermally treated with Na2CO3 aqueous solution at 100 degrees C, 150 degrees C and 200 degrees C for 72 h. After hydrothermal treatment, the cancellous bone-like macroporous structure of the alpha-TCP foams was maintained. However, microscopic morphology of the foams' frame significantly changed after the 72 h treatment period. The smooth surface of alpha-TCP foam disappeared and the whole surface was covered with plate-like deposits. The plate-like deposits treated at 150 degrees C and 200 degrees C had smooth surface while those treated at 100 degrees C were constructed from spherical particles of approximately 200 nm in diameter. The results of X-ray diffraction and Fourier transform infrared analysis showed that alpha-TCP was completely converted to CAp and the crystallinity of CAp prepared at 100 degrees C was significantly lower than those prepared at 150 degrees C and 200 degrees C. Hydrothermal treatment of alpha-TCP foam at 100 degrees C allowed the formation of low-crystalline CAp foam but complete conversion needs a longer treatment period.

Effects of sintering temperature over 1,300 degrees C on the physical and compositional properties of porous hydroxyapatite foam.

Porous hydroxyapatite (HAP) foam permits three-dimensional (3D) structure with fully interconnecting pores as well as excellent tissue response and good osteoconductivity. It is therefore thought to be a good candidate as scaffold material for bone regeneration and as a synthetic bone substitute material. To fabricate better porous HAP foam, improved physical and structural properties as well as higher osteoconductivity are desired. In the present study, the effects of sintering temperature on the physical and compositional properties of porous HAP foam were evaluated by employing high sintering temperature starting at 1,300 degrees C up to 1,550 degrees C. The mechanical strength of porous HAP foam increased with sintering temperature to reach the maximum value at 1,525 degrees C, then decreased slightly when sintering temperature was further increased to 1,550 degrees C. Alpha tricalcium phosphate (alpha-TCP) was formed, and thus the porous HAP foam became biphasic calcium phosphate. Biphasic calcium phosphate consisting of both alpha-TCP and HAP had been reported to show higher osteoconductivity than HAP alone. We therefore recommend 1,500-1,550 degrees C as the sintering temperature for porous HAP foam since this condition provided the most desirable physical properties with biphasic calcium phosphate composition.